Complex eastern Indonesia poses exploration challenges

John Clure
Technical Outsourcing
Letcombe Regis, Oxfordshire, England

The Indonesian archipelago tectonically comprises the collision between Southeast Asia and Australasia, the better explored western Indonesia being the Asian section and Eastern Indonesia consisting of the Australasian section and the collision zone between the two.

Two biologically defined lines, the Wallace Line and the Lydekker Line, bound this collision zone (Fig. 1 [131,181 bytes]). The Wallace Line runs down the Makassar Straits between Borneo and Sulawesi and then between Bali and Lombok. West of this line the fauna are strictly Asian. The Lydekker Line runs between Halmahera and Irian Jaya, east of the Banda Arc and south of Timor. East of this line the fauna are strictly Australasian.

It is these divisions, geological and biological, which create the three distinct environments for petroleum system development. The Western Indonesian evolution is better known due to the economic success of the area.

Western Indonesia consists of the Sunda Craton, which is made up of Mesozoic economic basement. The collision of the Indian sub-continent with southern Asia brought about the development of many rift basins around the craton during the Tertiary, and it is these Tertiary basins that provide the vast majority of Indonesia's petroleum resources. The majority of the western basins are mature explorationwise, and thus the major discoveries have been made. Future exploration there is dependent on small features or stratigraphic plays.

Eastern Indonesia has not had as much success and is less well explored. It thus still has many large untested features, but with higher exploration cost and higher risk.

Eastern Indonesia structure

East of the Lydekker Line tectonically is part of the Australian Craton, and thus the geology is more related to Australia than to Asia. Between the two lines are numerous islands comprising continental fragments, presumably broken off from the Australian Continent, in addition to volcanic arcs.

Approximately half of the Indonesian basins are in Eastern Indonesia, and many of them remain undrilled. The Australian Craton part of eastern Indonesia consists of an extension of the Northwest Shelf with its associated rifted margin plays as well as the collision zone between the Australian Plate and the Pacific Plate, which forms the mountains of Irian Jaya and Papua New Guinea.

To the south of these mountains is the Akimeugah basin, which is made up of a fold-belt with overthrust and subthrust plays, a foredeep, and a tensed foreland. To the north of the fold-belt the basins are located on the Pacific plate and thus different regime. To the west the Birds Head contains a similar sequence to the Akimeugah basin separated by the flexure of the Lengguru fold-belt and the addition of the late Miocene Kais limestone. The Birds Head, or Kepala Berung as it is known in Indonesian, changes at the Sorong Fault Zone from the Australian Plate in the south to the Pacific Plate in the north.

The islands of the collision zone consist of the Volcanic trend that makes up the Lesser Sunda Islands and the Banda Arc, the accretionary wedge that makes up Timor, the complex of various fragments of the Molucca islands including Seram, with Bula oilfield, and the larger island of Sulawesi. Sulawesi is made up of a series of continental fragments welded together, each with its own unique geology. The southern leg may have once been part of the Sunda Craton with its small gas discoveries and enticing oil seeps. Other parts of the island comprise paleo-volcanic arcs and metamorphic zone as well as pods of sediments.

Just off Sulawesi's southeast leg is Buton island, which has attracted much attention in the past due to its large tar pits. This all results in a much more complicated and variable geology than is seen in Western Indonesia and is thereby filled with many unknowns. This article will cover the more important plays.

Akimeugah basin, Irian Jaya

The Akimeugah basin was initially the northern part of the Australian continental shelf during the Mesozoic (Fig. 2 [124,166 bytes]). This has been complicated by the oblique collision with the Pacific Plate, which resulted in the formation of the thrust belt during the late Tertiary.

The plays in the basin are Mesozoic and are related to the foredeep in front of the fold belt. The source rock is probably Upper Jurassic, the same as the Northwest Shelf of Australia. During the Mesozoic this area used to be a continental margin created by Jurassic rift and drifting. Jurassic sediments would be restricted to the foredeep and to the thrust belt, where it has been identified in river sections.

The reservoir is Cretaceous Wonowogi sandstones. This is sealed by the thick overlying Cretaceous Pinya shale. The area appears to have all the ingredients for success with a thick reservoir, a good thick seal, a Jurassic source rock, and a 1,000 mile fold belt.

Three plays comprise the overthrust folds, sub-thrust structures, and fault related features in the tensed foreland south of the fold belt. Discoveries have been made in the Papua New Guinea part of the fold belt, predominantly gas-condensate, but the search has been harder on the Indonesian side.

Why the Papua New Guinea trend does not continue could be related to the change in the type of thrusting and thus effecting the migration pathways. This change in thrusting style from thin-skinned thrusting in the west, to basement related thrusting in the east, occurs where the mountain range changes from northwest-southeast trend to an east-west trend at the Baliem Valley (at 140° E. Long.). To the west the basal detachment is within or above the Mesozoic Kembelangan group in the disturbed belt in the foothills. Farther back in the highlands some of the thrust sheets contain Paleozoic to Tertiary sections. To the east the thrusts are basement cored.

The exploration of this acreage started with Shell under the guise of NNGPM in the sixties with geological field parties and the drilling of Jaosakor-1 and Aripu-1. Both of the wells were located close to rivers and were not structurally sited. These wells were on the foreland basin and well isolated from the Jurassic seaway and hence source rock.

A large part of this acreage from the Arafura Sea to the Papua New Guinea border was awarded in the sixties as Mamika Eilanden. A group comprising Amoseas, Conoco, and Total explored this large area, consisting of vast tracts of tropical rain forest and terrain from mountains down to swamps. A coarse grid of seismic was shot and a couple of shallow wells drilled on the foreland in search of possible reef features, Kumbai Satu-1 and Kuruwai-1, without success.

The area was renewed under the operatorship of Conoco as Nauka, which contained the vast majority of the thrust belt. The only well to be drilled in this phase tested the thrust play and was Noordwest-1. However, Noordwest-1 was finished as the lease expired with the encounter at TD of oil shows, but the well was lost and not logged.

These oil shows obviously brought further encouragement, and a similar lease was renegotiated as Warim. Further geological field work and seismic delineated a couple of overthrust features, which where drilled (Cross Catalina-1 and Sande-1). Sande-1 well penetrated the steep limb of the fold and never got out of the Pinya formation. The Cross Catalina well encountered oil shows in the Wonowogi sandstone, but it was extremely tight.

Since then exploration has been focused on the foreland, where the reservoir is shallower and Digul was drilled with shows. Conoco's recently drilled Kau-1, also a foreland play, is rumored to be a gas-condensate discovery in a thick Jurassic sandstone. However, it appears they junked the hole without a thorough evaluation. This is certainly an exciting area, but at over $20 million a hole it is not for the light of heart.

The sub-thrust play has not been explored, the fears being that the one well testing the reservoir in the overthrust had a tight reservoir and the question as to whether the sub-thrust features are seismic pull ups under the thrust sheet. There are so few wells, in an area so large, with so little seismic coverage that there still could be elephants hiding out there, to be found by an explorer with intestinal fortitude and a lot of money. The foredeep stretches a short way offshore into the Arafura Sea. The Maxus Kola-1 well would have been in an ideal location to test the "play" with an onshore and hence cheaper well. However, this did not prove successful.

Irian Jaya birds head

Kepala Berung is one of the few oil producing areas of Eastern Indonesia and is likely to become one of Indonesia's major gas producers, assuming that the oil can be sold for U.S. dollars (Fig. 3 [94,872 bytes]).

Initially NNGPM explored this area and discovered the Tertiary Kais/ Klasaman reefs of the Salawati basin with Klamono-1 and tried for mirror images on the other side of the basement high in the Bintuni basin. This was not as successful with the discovery of structural carbonate features at Wasian and Mogoi. These had poor quality Kais reservoirs, being very argillaceous.

Trend followed up with numerous reef discoveries in the Salawati basin followed by the Phillips discovery on Salawati Island. Conoco and Total moved in on the mirror image play in the Bintuni basin. Many dry holes in the Kepala Berung A & B blocks led to the knowledge that the Kais formation was flushed. The rain in the mountains to the north was passing through the prospects and keeping them clean of hydrocarbons, the oil of Mogoi and Wasian being trapped by the poor quality Sekau member of the Kais formation.

The so-called Sekau formation is brought about by the drop in sea level and an increase in clastic sediment input in this area at the end of the period of Kais deposition, resulting in some of the high areas having the more argillaceous limestone, and thus the oil was bypassed when the water started flushing through after the uplift and exposure of the Kais in the mountains.

This did not occur to the same extent in the Salawati basin due to faster basinal subsidence. Thus, basin subsidence kept ahead of fall in sea level. However, one small discovery was made at Wiriagar adjacent to where NNGPM had encountered oil in earlier Wiriagar wells. This field, again due to poor reservoir quality, was not long lasting.

The focus of operators in the area then turned to the Mesozoic as this was felt to be more mature for oil and probably contained the source for the oil so far discovered in the Kais. The initial problem was that the Jurassic section was wedging out to the north, and the deepness of the Bintuni basin could result in the generation of gas as opposed to oil.

The gas theory was proven correct with the discovery by Occidental of Roabiba-1. Arco then picked up the deep rights for Wiriagar, farmed into the offshore Bintuni block, drilled Wiriagar Deep, and made a gas discovery. Further drilling has proven up even more gas at Vorwata. The project is now named Tangguh and comprises both discoveries.

Seram and Halmahera

Seram, although situated on the northern part of the Banda Arc, was initially part of the Australian plate, and the Mesozoic to late Miocene sediment sequence is correlatable with the Northwest Shelf (Fig. 4 [99,870 bytes]).

The late Miocene to present sediments were deposited since this fragment separated from the Australian plate and is strongly influenced by the interaction between the Pacific Plate, the Australian Plate, and the Asian Plate. The micro-continent that contains the island of Seram is part of a group of microcontinental fragments bounded to the north by the Sorong fault and to the south by the Tarera-Aiduna fault. These two strike slip faults are moved by the oblique convergence of the Pacific Plate and the Australian Plate. The Misool-Birdshead Plate is currently being subducted under Seram at the Seram trough, and it is this continental fragment to continental fragment collision that has brought about the formation of the thrust belt that makes up the mountains on Seram.

This is an area that has been explored for oil since the last century. Oil seeps were recorded as early as 1865, and the oil was first produced in 1897 by BPM from Bula field. This field initially produced 200 b/d from Pleistocene sands and peaked in the 1930s at 2,000 b/d.

Bula field is believed to have had initially 200 million bbl of oil in place and has so far produced about 15 million bbl. However, the oil is generated in the Triassic and Jurassic Kanikah formation and the Saman-Saman limestone and migrated via faults into the Pleistocene. It is the search for the Mesozoic play that has resulted in the 1993 discovery of Oseil-1. This well established about 20 million bbl in the fractured Jurassic limestone (Manusela formation) and is being delineated this year. The Saman-Saman limestone is the basinal equivalent of the Manusela formation, and they thus make an ideal source/reservoir pair. Oseil-1 tested over 6,000 b/d from two zones within the limestone.

North of Seram is a Sulawesi look-alike called Halmahera. This is predominantly volcanic and metamorphic and will thus not have much in the way of a clastic source. There are basins situated between the arms of this island; however, any play would be dependent on carbonate development. The size of these basins would also put a risk on the volume of potential source rocks, and there could be a risk of maturity.

Sulawesi, nearby islands

Sulawesi and nearby islands have been interesting due to numerous hints, but not anything big yet.

Offshore to west of the southern leg in the region of the Spermonde shelf there is a rift, which if this part of Sulawesi were part of the Sunda landmass would have been connected to a paleo-drainage system that comes from central Borneo. This rift contains some very large structures, but in water depths in the thousands of meters, which can prove hard on the economics. Onshore small gas discoveries where made in Miocene carbonates.

North of this in the Kalosi area the hilly, forested terrain was explored by Arco due to numerous Eocene oil seeps being found at outcrop in some of the valleys. The Synrift-Toraja formation that overlays basement contains the best source rocks, which are fluvial-deltaic coals, and the fluvial-deltaic sandstones of this formation are the primary reservoir objective with the interbedded shales providing the sealing mechanism. However, it is rumored that drilling of Loka-1 failed to find a reservoir.

This play is complicated by the imbricate nature of the Majene and Kalosi thrust and fold belts. Thus, the major risks of this play are that the distribution of the seal and reservoir and the multiphase intense tectonism of Sulawesi may have breached any accumulations.

The northeast arm of the island comprises a collision zone between the Bengai Sula Microplate. This microplate contains Mesozoic sediments, but it is uncertain if there is enough sediment to provide oil maturity or quantity of source rock. However, the northeast arm of Sulawesi contains these sediments also and much thrusting created by the collision.

The thrusting is of imbricate intensity and thus far too chopped up for economic sized structures. However, offshore there was a carbonate build-up play that was tested by Union Texas. They discovered the carbonate, with hydrocarbons at Tiakka, but it was in fractured limestone. Further evaluation of this feature indicates that the limit or interconnectivity of the fractures was a restriction.

To the southeast is the island of Buton with its tar pits which have been mined for many years. It was the presence of this tar that demonstrated the generation of large amounts of oil on this island. The first company to drill for oil on this island was Gulf with Bale-1, Bulu-1, and Sampolakosa-1. Then Conoco evaluated the island and drilled one well near the coast at Jambu-1.

The problem with the Buton play is the seal. The oil can be seen as tar pits and blocking the pores of sands at the surface, and although oil is all around to see there is none left in liquid form that has been found so far.

The Banda Arc

The continuation of the Northwest shelf into Indonesian waters has always provided interest to the industry with large Mesozoic fault blocks being observed from the east of Tanimbar north to Irian Jaya. However, concerns arise with regards the faults, which appear to go to the surface, and thus could provide a conduit for any hydrocarbons to escape.

An examination of the seismic data across Jabiru on the Northwest Shelf of Australia also reveals faults going all the way to the surface. The question is whether the faults are sealing or is the trap at Jabaru in fact stratigraphic, being an angular unconformity. This could also apply to the Indonesian side. This play has been tested on the Indonesian side by Promet with Koba-1 and by Union Texas with Barakan-1 with no sign of hydrocarbons.

The islands of Kai and Tanimbar are highly imbricate, and thus a major problem to map and any traps would be complicated and highly compartmentalized.

Timor

The island of Timor is highly thrusted and compartmentalized, which again provides problems with mapping and restricts the size of each fault block.

However, Timor contains many oil seeps demonstrating the presence of oil in the area. It is this presence of tempting seeps that keeps drawing the punters. Offshore quickly plunges into a trough; however on the other side of the trough there is a possibility of a Northwest Shelf play.

Conclusions

In Eastern Indonesia, provinces that are adjacent to each other are not necessarily related and may have even originated on an entirely different plate.

In fact the geology of Eastern Indonesia is the subject of much research by many organizations and is the subject of much geological controversy due its complex nature.

The focus of attention has been has been on Irian Jaya in for many years, and that area continues to attract attention. Proximity to the Northwest Shelf always provides interest. The oil seeps on Timor and Sulawesi and the tar of Buton provide an inducement.

Large areas of Eastern Indonesia are covered by the Banda Sea, which is oceanic crust and not prospective. The hydrocarbons continue to be found in the traditional basins of Salawati, Bintuni, and Seram, and people will also continue to chase the seeps elsewhere.

Acknowledgments

I acknowledge the help and support of other members of Technical Outsourcing as well as Steven Courteney of Wairarapa Geological Services, who provided the base art work for the figures and helpful tips.

The Author

John Clure has been involved with oil and gas exploration for 30 years, predominantly with major oil companies. He has been involved with all aspects of the industry ranging from new ventures and regional studies to development and from operations to prospect generation. He has carried out studies on all continents and dealt with sediments ranging from Infra-Cambrian to Pleistocene and with carbonates and clastics. He has worked in Indonesia on numerous occasions, for a total of 12 years. He is presently a partner in a geological consultancy. E-mail: [email protected]